Hydraulic turbines are very useful to support the electrical grid stability when the demand is quickly varying. They convert the potential energy of the water into rotating mechanical energy, which is then converted to electrical energy by the generator.
FIG. 1 represents a multi-group hydroelectric power plant with a hydraulic turbine 1, a draft tube 2, a generator 3, a step-up transformer 4 and an electrical network (grid) 5. With the growing integration of intermittent renewable sources of electricity such as photovoltaic panels and wind turbines, hydroelectric power plants are an efficient way of supporting the grid but this leads to new challenges in their design and operation. Two of these challenges are a minimum time start-up of the turbine from standstill, and operating it at off-design conditions, such as partial load. According to embodiments of the present invention, it is disclosed a novel control algorithm particularly for the operation at partial or full load.
At partial load, more precisely when the flow through the turbine is a fraction of the optimal flow, reaction turbines exhibit a helical vortex rope in their draft tube resulting from the swirling flow exiting the runner, as shown in FIG. 2.
The interaction of this vortex rope with the draft tube can lead to a pressure perturbation propagating in the entire hydraulic system with a frequency in the range of 0.2 to 0.4 the turbine rotational frequency. Embodiments of the present invention address the technical disadvantages related to effects of the pressure perturbation on the produced electricity.
Indeed, these oscillations of pressure are converted in torque oscillations by the turbine and eventually in oscillations of active power transmitted to the network. In some cases, these oscillations of electric variables are unacceptable for operators because they don't comply with network specifications, described in the grid codes. These specifications of network operators describe the performances a power plant needs to have to be connected to the grid. The level of oscillations for the active power and voltage assume in this scenario a prominent relevance, as they need to be respected to avoid an excitation of the electrical grid modes of oscillation.
Traditionally, the control loop of the hydraulic turbine is decoupled from the excitation controller of the generator due to the difference in time response of the two subsystems, the generator having a faster response time. In the case of low frequency hydraulic oscillations, an interaction can appear between the hydraulic and electric subsystem thus worsening the oscillation. For this reason, the controller according to embodiments of the present invention rely on both electrical and hydraulic subsystems as a whole.
Most of the work on the control of hydraulic turbine has been focused on developing algorithms to improve robust performance of the controllers. The two main challenges of a hydraulic turbine governor are the non-linearity's of the turbine characteristic and the unstable zeros. Several control designs have been proposed in, including optimal PID gain scheduling, adaptive algorithms, robust control considering plant uncertainties, and more recently robust PID design where the robust performance of the PID controller is favourably compared to a more sophisticated H∞ controller. All these designs teach or suggest a linear model of the turbine, either a linearized model from the turbine characteristics or an ideal model developed in.
Additionally, an approach of simultaneously controlling both the turbine wicket gate opening and the generator excitation voltage has been developed in the field. The design is based on an ideal nonlinear model of the turbine and a full 7-order nonlinear model of the synchronous machine to improve stability after large electrical transients, for example a short-circuit, or a lightning bolt.
The concept of damping inter area oscillations using a power system stabilizer (PSS) for synchronous generator has been used in the field to design a power system stabilizer using the hydro governor system. The resulting approach provides much better damping of the low frequency inter area oscillations during poor grid conditions.
The concept of reducing the effect of the vortex rope on the electric power with the PSS on the synchronous generator only has also been explored. While the active power oscillations originating from hydraulic pressure fluctuations are attenuated, they are amplified on the reactive power and the voltage.
Considering that the vortex rope hits the elbow of the draft tube in the centre of it, the turbine draft tube has been modelled with two equal-length pipes and a pressure source in the centre. This model was developed to study the system stability when it is subject to the partial load vortex rope. The studied system consists of four hydro-electric groups connected to the electrical network.